1. Field of the Invention
The invention is related to magnetic storage devices, and in particular, to a hard disk drive including a current perpendicular to plane (CPP) differential magnetoresistance (DMR) read head design using a current confinement structure proximal to an air bearing surface (ABS).
2. Statement of the Problem
Of the many magnetic storage devices, a hard disk drive is the most extensively used to store data. The hard disk drive includes a hard disk and an assembly of write and read heads. The assembly of write and read heads is supported by a slider that is mounted on a suspension arm. When the hard disk rotates, an actuator swings the suspension arm to place the slider over selected circular data tracks on the hard disk. The suspension arm biases the slider toward the hard disk, and an air flow generated by the rotation of the hard disk causes the slider to fly on a cushion of air at a very low elevation (fly height) over the hard disk. When the slider rides on the air, the actuator moves the suspension arm to position the write and read heads over selected data tracks on the hard disk. The write and read heads write data to and read data from, respectively, data tracks on the hard disk. Processing circuitry connected to the write and read heads then operates according to a computer program to implement writing and reading functions. One type of read head is a differential read head. In a differential read head, two separate magnetoresistance (MR) sensors are separated by a non-magnetic electrically conductive gap. Besides being separated by the conductive gap, the MR sensors are fabricated such that the magnetic moment of each of the MR sensors in the read head are orthogonal to each other. Because the magnetic moment of each of the MR sensors are orthogonal, stray magnetic fields interacting with the MR sensors (i.e., common mode magnetic noise) is cancelled out in the read head. This occurs because as the resistance in the first MR sensor decreases due to the common mode magnetic noise, the resistance in the second MR sensor increases. The net effect across both MR sensors in the read head is to substantially cancel out the effect of the common mode magnetic noise.
During a reading function, the read head passes over magnetic field transitions on the rotating hard disk. When the read head encounters the magnetic transitions, they interact with the read head to modulate the resistance of the MR sensors within the read head. In order to generate a read signal from the read head, a sense current is injected into the MR sensors. The read signal is generated by the sense current and the modulating resistance within the MR sensors generating a modulating voltage. Circuitry within the hard disk drive senses the voltages in the read head to generate read signals from the read head. The resulting read signals are used to decode data encoded by the magnetic transitions of the data track.
In order to increase the amount of storage available on the hard disk drive, the magnetic transitions are typically are placed closer together. One problem with placing the magnetic transitions closer together is that the strength of the magnetic fields used to generate the magnetic transitions tends to be smaller in order to prevent unintended interactions between one magnetic transition and another. When the strength of the magnetic fields is reduced, the amount of resistance modulation within the MR sensors generally decreases due to the lower magnetic field strengths. The decrease in the amount of resistance modulation in the MR sensors reduces the amount of read signal available from the read head for a given sense current. Another problem with placing the magnetic transitions closer together is that stray magnetic fields from adjacent magnetic transitions can increase the amount of common mode magnetic noise that the read head encounters. Both the reduced read signals generated and an increase in common mode magnetic noise are problems encountered when increasing the storage available on the hard disk drive.